rcu: Prevent force_quiescent_state() memory contention

Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field.  This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.

The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node.  In addition, this
functionality is not used on fastpaths.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

2 files changed, 38 insertions, 10 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 723e2e723074..43d57a17fcc5 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -61,6 +61,7 @@
 /* Data structures. */
 
 static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
+static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
 
 #define RCU_STATE_INITIALIZER(sname, cr) { \
         .level = { &sname##_state.node[0] }, \
@@ -1807,16 +1808,35 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 static void force_quiescent_state(struct rcu_state *rsp)
 {
         unsigned long flags;
-        struct rcu_node *rnp = rcu_get_root(rsp);
+        bool ret;
+        struct rcu_node *rnp;
+        struct rcu_node *rnp_old = NULL;
+
+        /* Funnel through hierarchy to reduce memory contention. */
+        rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
+        for (; rnp != NULL; rnp = rnp->parent) {
+                ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
+                      !raw_spin_trylock(&rnp->fqslock);
+                if (rnp_old != NULL)
+                        raw_spin_unlock(&rnp_old->fqslock);
+                if (ret) {
+                        rsp->n_force_qs_lh++;
+                        return;
+                }
+                rnp_old = rnp;
+        }
+        /* rnp_old == rcu_get_root(rsp), rnp == NULL. */
 
-        if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
+        /* Reached the root of the rcu_node tree, acquire lock. */
+        raw_spin_lock_irqsave(&rnp_old->lock, flags);
+        raw_spin_unlock(&rnp_old->fqslock);
+        if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
+                rsp->n_force_qs_lh++;
+                raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
                 return;  /* Someone beat us to it. */
-        if (!raw_spin_trylock_irqsave(&rnp->lock, flags)) {
-                rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */
-                return;
         }
         rsp->gp_flags |= RCU_GP_FLAG_FQS;
-        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+        raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
         wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
 }
 
@@ -2704,10 +2724,14 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
 static void __init rcu_init_one(struct rcu_state *rsp,
                 struct rcu_data __percpu *rda)
 {
-        static char *buf[] = { "rcu_node_level_0",
-                               "rcu_node_level_1",
-                               "rcu_node_level_2",
-                               "rcu_node_level_3" };  /* Match MAX_RCU_LVLS */
+        static char *buf[] = { "rcu_node_0",
+                               "rcu_node_1",
+                               "rcu_node_2",
+                               "rcu_node_3" };  /* Match MAX_RCU_LVLS */
+        static char *fqs[] = { "rcu_node_fqs_0",
+                               "rcu_node_fqs_1",
+                               "rcu_node_fqs_2",
+                               "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
         int cpustride = 1;
         int i;
         int j;
@@ -2732,6 +2756,9 @@ static void __init rcu_init_one(struct rcu_state *rsp,
                         raw_spin_lock_init(&rnp->lock);
                         lockdep_set_class_and_name(&rnp->lock,
                                                    &rcu_node_class[i], buf[i]);
+                        raw_spin_lock_init(&rnp->fqslock);
+                        lockdep_set_class_and_name(&rnp->fqslock,
+                                                   &rcu_fqs_class[i], fqs[i]);
                         rnp->gpnum = 0;
                         rnp->qsmask = 0;
                         rnp->qsmaskinit = 0;
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 7fb93cedc76a..8f0293ce1517 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -202,6 +202,7 @@ struct rcu_node {
                                 /*  per-CPU kthreads as needed. */
         unsigned int node_kthread_status;
                                 /* State of node_kthread_task for tracing. */
+        raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 } ____cacheline_internodealigned_in_smp;
 
 /*